1. Field of the Invention
The present invention relates generally to telecommunications, and particularly to a method and system for evaluating the quality of voice signals.
2. Technical Background
Telephone connections have always been subject to impairments in the form of noise, attenuation, distortion, cross-talk and echo. Such impairments are particularly common to analog portions of the network, such as subscriber loops and frequency domain multiplexing equipment. Digital transmission alleviates any of these problems but also introduces quantization noise and distortions due to bit errors in the digital signal. However, even with perfect digital transmission applied to long haul transmissions, a typical telephone connection includes many analog components, wherein impairments can occur.
A poor connection or malfunctioning piece of equipment can produce conditions that a telephone customer will find objectionable or intolerable. When there is a high incidence of poor connections, customers may complain to the service provider or to a regulatory authority, or simply change long distance carriers. Thus, perceived quality of telephone connections is a major factor affecting the reputation and marketability of long distance telephone services.
To guard against poor quality, service providers have developed methods to obtain objective quality measurements upon a line, piece of equipment, or an end-to-end telephone connection. These measurements can help the service provider detect and gauge impairments, pinpoint weak elements, and correct deficiencies that degrade user perception of quality. The effects of extreme fault conditions on user perception of quality is clear. There are easily discernable thresholds for “no effect” and “substantial degradation” conditions. Unfortunately, for intermediate objective quality measurements, there is no clear division between values representing acceptable and unacceptable voice connection quality.
In one approach that was discussed in the parent application, mappings were created between objective measurements and user perceived quality ratings. A set of objective characteristics for a telephonic connection were obtained. Test signals were produced by varying each element in the set of objective characteristics, and by varying various combinations of the objective characteristics. Each test signal was subjectively rated by a large group of evaluators using a “no impairment,” “some impairment,” or “much impairment” rating system. These data mappings were synthesized by constructing a look-up table. The quality of a telephonic connection can be assessed using this mapping system. When a set of objective measurements for a telephonic connection are obtained, the look-up tables can be used to provide a network analyst with an estimate of the perceived quality level for the connection. This mapping system is a powerful tool for analyzing reported impairments, or for testing new network plant or equipment before deployment.
However, there are several shortcomings associated with the approaches described above. Although look-up tables can be used to store the mapping data, it would be difficult to use them to provide a device capable of real time processing to produce measurements. The use of a look-up table that contains all of the mappings would substantially increase memory requirements and necessitate a prohibitive amount of processing. The CPU in such a device would be required to perform a large number of operations. Because the contents of the look-up table are discrete mappings, the accuracy of a real time estimate may be poor.
Thus, what is needed is a device that uses an analytical representation of the mapping tables. In other words, the mapping data in the data tables would be modeled as a continuous analytical transform. This would significantly reduce costs, and the size of a resultant voice quality estimation device. By using a continuous mathematical function, CPU processing time would be significantly reduced, allowing the device to obtain a real-time estimate of likely user perception of a given connection in terms of the user perception rating system described above. Finally, a reprogrammable device is needed that can be updated as more accurate mapping data is obtained.